She meets Steve Rogers while he was puny and during the training period she got to know him, but it wasn't until after the experimentation where she gained an attraction for him.

She later helped Steve Rogers perform a one man rescue mission to free the captured Soldiers from a Hydra base. After that she assisted Captain Rogers in any way she can.

She was the one who helped test Captain Roger's shield in a fit of rage from seeing him kiss another girl she pulled up a gun and shot several times at the shield proving it worked and scaring both Rogers and Howard Stark in the process.

At the end of the movie she talks to Rogers as he is forcing the enemy plane to crash land planning a date. She is last seen looking over a file of Captain Rogers before setting it down and leaving it behind.

After Steve died, Carter joined a New York branch of the S.S.R., hoping to recieve field missions, but women were looked down upon in the 1950's, and Peggy mostly ordered the men lunch and brought them coffee. When Howard Stark disappeared when many of his inventions were stollen, he turned to Peggy to help him clear his name. With the help of Howard's butler, Edwin Jarvis, Peggy uncovered a secret plot run by a Russian organization called Leviathan. This included Leet Brannis, Johann Fennhoff, and Dottie Underwood, the last of which trained in the Black Widow Program. She took down their plot and cleared Howard Stark's name, becoming an active field agent for the S.S.R.

Later on, Carter moved to L.A. to work for S.S.R. there.

In a deleted scene from the Avengers it is revealed she did not die while Steve Rogers was in the ice.

In Captain America: The Winter Soldier, Steve visits Peggy, who now suffers from Alzheimers. She is very happy to see Steve, but suddenly acts as if Steve had never visited her before, telling him it had been "so long", to which he replied "Well I couldn't leave my best girl. Not when she owes me a dance."